High neutron absorbing refractory compositions of matter and methods for their manufacture

ABSTRACT

Neutron absorbing refractory B 4  C-Gd and Gd 2  O 3  -Gd cermets, B 4  C-Gd and Gd 2  O 3  -Gd metal-matrix composites, and B 4  C-Gd 2  O 3  ceramic-ceramic composites can be manufactured by applying fundamental thermodynamic and kinetic guidelines as processing principals. 
     Three steps are involved in the fabrication of these new compositions of matter. First, the starting materials are consolidated into a compacted porous green body. Next, the green body is densified using the appropriate method depending on the class of material sought: cermet, metal-matrix composite, or ceramic-ceramic composite. Finally, either during the densification process or by subsequent heat treatment, new phase evolution is obtained via interfacial chemical reactions occurring in the microstructures. 
     The existence of a new phase has been identified in B 4  C-Gd and B 4  C-Gd 2  O 3  composites.

This application is a division of application Ser. No. 07/591,094, filedOct. 1, 1990, now U.S. Pat. No. 5,156,804.

BACKGROUND OF THE INVENTION

This invention relates generally to boron carbide-gadolinium oxide,boron carbide-gadolinium, and gadolinium oxide-gadolinium compositionsof matter and more particularly to boron carbide-gadolinium oxideceramic composites and boron carbide-gadolinium and gadoliniumoxide-gadolinium cermets or metal-matrix composites.

U.S. Pat. No. 4,605,440 by Halverson, Pyzik and Aksay, U.S. Pat. No.4,704,250 by Cline and Fulton, and U.S. Pat. No. 4,718,941 by Halversonand Landingham all pertain to boron carbide-reactive metal compositesand their manufacture. However, these patents do not show specific boroncarbide-gadolinium compositions or methods for producing directly usableconsolidated bodies thereof. U.S. Pat. Nos. 4,826,630 and 4,474,728 byCarlson and Radford, U.S. Pat. No. 4,744,922 by Blakely and Shaffer,U.S. Pat. No. 4,671,927 by Alsop, and U.S. Pat. Nos. 4,657,876 and4,636,480 by Hillig describe various neutron absorbing materials.

The field of nuclear physics has matured and created a revolutionaryimpact on modern history. The applications of research inneutron-induced reactions appear both in other areas of fundamentalresearch and in the practical areas of nuclear energy production.

The ultimate fate of a free neutron liberated through some reaction iseither absorption by a nucleus, or transformation by beta decay. Thelatter process is so weak as to be negligible for practicalapplications. By a wide margin, the most important absorption process ina non-fissile nucleus is radiative capture.

Neutron capture can occur over nine orders of energy magnitude, slow orthermal neutrons from as low as 10⁻² eV to fast neutrons as high as 14MeV. Different atomic mechanisms are associated with the radiativecapture of each. Neutron capture processes have lifetimes ranging from10⁻²² seconds (thermal capture) to as long as 10⁻¹⁵ seconds (fastcapture).

For practical applications involving slow neutrons, it is sufficient towork with the statistical theory of radiative capture. This topic ishighly complex; however, qualitative consideration of materials can bemade by initially examining their average capture cross sectionproperties.

The nuclear cross section of a material is a measure of the probabilityof a particular process. In the case of radioative capture, the capturecross section is expressed as σ, and is the effective target area of thenucleus with which a neutron must interact to produce a given reaction.The unit of σ is the barn (1 barn=10⁻²⁴ cm²). Absorption cross sectionsfor thermal neutrons range from 4.6×10⁻⁴ barn for deuterium to 3.3×10⁶barns for xenon¹³⁵.

To protect nuclear reactor operating personnel against damagingbiological effects of neutrons and gamma rays, shielding is requiredaround nuclear reactors. Neutron and gamma ray fluxes in the range of10¹³ to 10¹⁴ must be attenuated to 10³ particles/cm² /sec to meettolerance radiation levels.

To attenuate gamma rays, which interact primarily with the orbitalelectrons of atoms, a material with high atomic number containing a highdensity of these electrons is required. Examples are lead, tungsten,depleted uranium, or concrete containing high-Z elements in the form ofscrap or heavy ore.

To attenuate neutrons, they must be slowed down and then absorbed.Hydrogenous materials such as water, concrete, or polyethylene areexcellent moderators. The slowed neutrons must be absorbed withoutproducing high-energy-capture gamma rays. This has historically beenaccomplished by using boron¹⁰ ; however, even with boron¹⁰ some gammashielding outside the neutron shield is generally required.

When a neutron is absorbed by the nucleus of an atom an exothermicprocess results and the compound nucleus reaches an excited energy statebetween 4 and 10 MeV, as determined by the center-of-mass kinetic energyand the rest-mass energy difference of the final and initial nuclides.This state decays by the emission of electromagnetic (gamma) radiation,leaving the compound nucleus in a lower energy state. Subsequentradiative decay of this and lower energy states, i.e., a cascade ofgamma rays, leaves the compound nucleus in its ground state, which may,or may not, be stable against alpha or beta decay (daughter products).

The inherent atomic processes associated with the radiative capture ofneutrons results in exothermic reactions that typically prohibit the useof hydrogenous materials because of their low-temperature phase changes;e.g., water boils at 100° C., polyethylene softens near 87° C.

Boron metal also has its drawbacks. It corrodes easily and is physicallyunstable under irradiation. Alloying to overcome these problems merelyreduces the boron content of the absorbing material. Because of theseconcerns, boron carbide has been used extensively as a neutron absorbingmaterial in various types of nuclear reactors for several decades.

Boron carbide exists as a homogeneous range of boron and carboncompositions between 9 and 24 at. % C. The most common stoichiometriesbeing B₄ C (B₁₂ C₃) and B₁₃ C₂, both of which are boron rich. Richerboron stoichiometries, B₈ C and B₂₅ C, are also known to exist; however,these are less favored thermodynamically. The high boron content andrefractory nature of B₄ C (melting point ≈2350° C.) made it a choicecandidate for high temperature neutron absorbing reactions.

The ideal neutron absorbing material would be light weight, refractory,not impart long-lived daughter products, be thermally shock resistant,of low density yet not too porous, be resistant to corrosion andoxidation, have high fracture toughness and high strength, and notpromulgate dust on delivery or while in use. Low cost, obviously, wouldbe another attractive advantage.

Boron carbide is refractory, has a specific gravity of 2.52, a modulusof rupture ≈300 MPa, and can be hot pressed into fully dense bodies.Boron carbide displays low fracture toughness, and also rapidly oxidizesabove 800° C. In addition, boron carbide's thermal shock resistance ispoor.

One way to increase the fracture toughness and thermal shock resistanceof B₄ C is by the addition of a metal phase, e.g., B₄ C-metal cermets ormetal-matrix composites. A cermet is defined as a ceramic-metalcomposite such that the final microstructure is ≧50 vol. % ceramicphases. A metal matrix composite is defined as a ceramic-metal compositesuch that the final microstructure is <50 vol. % ceramic phases. Theceramic phases can be the initial starting ceramic materials or reactionproducts that result from chemical reactions between two ceramic phasesor between ceramic and metal phases.

Another way to increase the fracture toughness and strength of B₄ C isthrough the introduction of another ceramic phase, e.g., B₄ C-Al₂ O₃, B₄C-TiB₂, and B₄ C-SiC composites. Although large increases in fracturetoughness and strength are generally obtained with the addition of ametal phase, the introduction of another ceramic phase can increasetoughness while maintaining the refractory nature of the composite.

One of the most appropriate metal phases to consider for the absorptionof neutrons is gadolinium, Gd. This metal also exists in the form of astable oxide known as gadolinium oxide, Gd₂ O₃. Gadolinium has thehighest nuclear capture cross section of any element known, σ≈40,000barns, compared to B¹⁰ with a σ≈4,000 barns.

Gadolinium offers mechanical and physical properties conducive tofabricating B₄ C-Gd cermets or metal-matrix composites, according to theinvention, that approach "ideal" neutron absorbing material conditions.For example, Gd is used as a burnable poison in shields and control rodsin nuclear reactors. It has a melting point of 1313° C., a boiling pointof ≈3000° C., and an α→β transformation temperature of 1235° C.Gadolinium tarnishes slightly in air at room temperature; however, evenat 1000° C. the oxidation rate is slow because of the formation of thetightly adhering oxide on the surface. It does not react with cold orhot water, but will react vigorously with dilute acids.

Gadolinium has the following mechanical properties: Tensile strength≈122 MPa, yield strength ≈17 MPa, elongation ≈47%, reduction in area≈58%, and an elastic modulus ≈56 GPa. It also has a thermal expansioncoefficient of ≈9×10⁻⁶ /° C.

Gadolinium's very low modulus of elasticity indicates it should besubstantially more resistant to thermal shock than B₄ C. By forming a B₄C-Gd composite, according to the invention, it should be possible toobtain a refractory body with very high neutron absorbing capability andgood thermal shock resistance. Because the specific gravity of Gd is7.90, the addition of B₄ C will also reduce the composite's weightsubstantially. Also, according to the invention, the reactions betweenGd and B₄ C during processing will introduce other ceramic phases intothe composite resulting in a higher overall fracture toughness.

According to the invention, similar material properties should beobtained by combining Gd₂ O₃ and B₄ C, or Gd₂ O₃ and Gd, to formceramic-ceramic composites, cermets, or metal-matrix composites. Forexample, Gd₂ O₃ has a specific gravity of 7.41 and a elastic modulus of≈130 GPa.

Accordingly, it is an object of the present invention to provideboron-carbide-gadolinium cermet compositions, boron-carbide-gadoliniummetal-matrix compositions, boron-carbide-gadolinium-oxide compositions,gadolinium-oxide-gadolinium cermet compositions, andgadolinium-oxide-gadolinium metal-matrix compositions.

It is also an object of the invention to provide methods for formingboron-carbide-gadolinium cermet compositions, boron-carbide-gadoliniummetal-matrix compositions, boron-carbide-gadolinium-oxide compositions,gadolinium-oxide-gadolinium cermet compositions, andgadolinium-oxide-gadolinium metal-matrix compositions.

It is another object of the invention to provideboron-carbide-gadolinium cermet compositions, boron-carbide-gadoliniummetal-matrix compositions, boron-carbide-gadolinium-oxide compositions,gadolinium-oxide-gadolinium cermet compositions, andgadolinium-oxide-gadolinium metal-matrix compositions with refractorymicrostructures, and methods for forming same.

It is a further object of the invention to provideboron-carbide-gadolinium cermet compositions, boron-carbide-gadoliniummetal-matrix compositions, boron-carbide-gadolinium-oxide compositions,gadolinium-oxide-gadolinium cermet compositions, andgadolinium-oxide-gadolinium metal-matrix compositions which are fullydense, and methods for forming same.

It is another object of the invention to provide articles of manufacturemade from boron-carbide-gadolinium cermet compositions,boron-carbide-gadolinium metal-matrix compositions,boron-carbide-gadolinium-oxide compositions, gadolinium-oxide-gadoliniumcermet compositions, and gadolinium-oxide-gadolinium metal-matrixcompositions.

It is also an object of the invention to provide methods for makingboron-carbide-gadolinium cermet compositions, boron-carbide-gadoliniummetal-matrix compositions, boron-carbide-gadolinium-oxide compositions,gadolinium-oxide-gadolinium cermet compositions, andgadolinium-oxide-gadolinium metal-matrix compositions, and articles ofmanufacture thereof at relatively low cost.

SUMMARY OF THE INVENTION

The present invention provides a whole spectrum of specific compositionsof boron-carbide-gadolinium, boron-carbide-gadolinium-oxide, andgadolinium-oxide-gadolinium composites which apply basic thermodynamicand kinetic principles to achieve these compositions. The inventionincludes a plurality of multiphase compositions, including fully densemicrostructures, and methods for selectively producing the desiredcompositions.

According to the invention, there are three major steps in the formationof these compositions of matter. First, the initial reactants must beproperly prepared. This is accomplished by selecting the appropriatestarting particle size distributions, dispersing and mixing theparticles, and consolidating the particles into a state that is readyfor step two.

Second, the capillarity thermodynamic criteria of achieving a rapidconsolidation through the wetting of the B₄ C or Gd₂ O₃ phase by the Gdmetal phase (Gd metal or Gd alloy) must be obtained in the case of theB₄ C-Gd cermets and Gd₂ O₃ -Gd cermets, respectively. Or, the conditionof plastic flow of the Gd metal (or alloy) around the B₄ C or Gd₂ O₃must be obtained in the case of B₄ C-Gd metal-matrix composites and Gd₂O₃ -Gd metal-matrix composites, respectively. Or, the solid-state and/orliquid-state rearrangement of oxide and carbide phases around each othermust be obtained in the case of B₄ C-Gd₂ O₃ composites.

The third step is to apply reaction thermodynamic criteria to theboron-carbide-gadolinium, boron-carbide-gadolinium-oxide orgadolinium-oxide-gadolinium compositions in order to achieve desiredreaction products in the microstructure. Through this step, it ispossible to take each respective composition of step two and react themto specific end products which result in different microstructures thanthose obtained in step two. In the case of the cermet or metal-matrixcompositions, it is also possible to completely react all of the Gdmetal or alloy thereof and any metastable phases which form during theseprocesses to achieve a composite material which is completely withoutany metal phase or any phase representative of the initial startingconstituents. That is, it is possible to start with a cermet ormetal-matrix composition and end up with a multiphase ceramic composite.

In general it is necessary to apply the kinetics of how theseboron-carbide-gadolinium, boron-carbide-gadolinium-oxide, andgadolinium-oxide-gadolinium compositions consolidate during the aboveprocesses in order to select the appropriate method of manufacture.Final consolidation involves the application of temperature to thesebodies such that the microstructural phases will flow together orsinter. It may also involve the application of pressure with temperaturein order to assure that fully dense final products are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of neutron capture cross section versus gadoliniumcontent in boron-carbide-gadolinium composites.

FIGS. 2A and 2B illustrate the wetting step in accordance with theinvention.

FIG. 3 is a graph of contact angle data for gadolinium metal in aboron-carbide substrate.

FIG. 4 is a vapor pressure curve for gadolinium.

FIG. 5 is a flow chart of the fabrication sequence for producingboron-carbide-gadolinium and gadolinium-oxide-gadolinium metal-matrixcomposites.

FIG. 6 is the phase equilibrium diagram for the GdBO₃ -B₂ O₃pseudooinary system.

FIG. 7 is a microstructure of a boron-carbide-gadolinium cermet.

FIG. 8 is a microstructure of a boron-carbide-gadolinium-oxidecomposite.

FIG. 9 is a microstructure of a gadolinium-oxide-gadolinium cermet.

DETAILED DESCRIPTION OF THE INVENTION

Gadolinium (or an alloy thereof) is a compatible metal phase with B₄ Cor Gd₂ O₃ ceramics and Gd₂ O₃ is a compatible ceramic phase with B₄ Cceramic for the development of B₄ C-Gd, B₄ C-Gd₂ O₃, and Gd₂ O₃ -Gdrefractory composites for neutron absorption because the starting phasesare reactive with each other.

Gadolinium and its oxide are terrestrially stable metal and ceramicphases, respectively. Gadolinium possesses the highest neutron capturecross section of any element known. This metal and its oxide is thethird most abundant of the lanthanide series of elements in the periodictable. Only cerium and samarium are more plentiful. Its abundance makesit a cost effective alternative to boron-based neutron absorbingmaterials.

Combining Gd or Gd₂ O₃ with B₄ C, or combining Gd₂ O₃ with Gd allows theuseful nuclear, thermal, and mechanical properties of each phase to acttogether to form refractory composites for practical nuclearapplications. In particular, these composites offer properties that canbe controlled through various processing routes to obtain high neutroncapture ability, reduced weight, thermal shock resistance, improvedfracture toughness, corrosion and oxidation resistance. Calculatedneutron absorption values (minimum and maximum) for B₄ C-Gd cermets andB₄ C-Gd metal-matrix composites are given in FIG. 1.

Potential applications of B₄ C-Gd, B₄ C-Gd₂ O₃, and Gd₂ O₃ -Gdrefractory composites include, but are not limited to, neutronshielding, control rods, burnable absorbers, secondary shutdown systems,spent fuel storage containers, transportation casks, personnelprotection, protection of electronics, and other waste handling orencapsulation applications requiring neutron attenuation. Thesecomposites can also serve as cathodes and anodes in long-life electronicdevices and laser systems.

The methods for forming B₄ C-Gd, B₄ C-Gd₂ O₃, and Gd₂ O₃ -Gd refractorycomposites are described in the following. The methods include threeprincipal steps, including (1) consolidation or preparation of thestarting materials, (2) densification by producing the rightcapillarity-thermodynamic condition in the case of systems processedabove the melting point of Gd, or by producing the correct plastic flowconditions in the case of systems processed below the melting point ofGd, or by producing the correct interphase diffusion and rearrangementconditions in the case of processing with non-metallic phases, (3)reacting the starting materials to produce the desired compositions.

CONSOLIDATION OF STARTING MATERIALS

Correct preparation of the starting materials is required to producefully dense microstructures or at least microstructures with negligibleporosity. Preparation and consolidation of the starting materialsinvolves three steps: Selection of the appropriate starting particlesize distributions, dispersion and uniform mixing of the appropriateparticles, and consolidation of these particles in a very homogeneousmanner.

Selecting the correct particle size distributions of the startingmaterials is important because it directly affects densification andreaction kinetics. That is, large particles will have much less surfacearea than small particles, making the available surface for chemicalreactions smaller. In addition, the selection of several differentparticle size distributions in combination can enhance the particlepacking density making microstructural rearrangement distancessubstantially smaller thereby promoting densification and reactionproduct formation.

Uniform mixing of the starting constituents can be performed bymechanical mixers or vibratory shakers; however, optimum uniformity isusually obtained through the use of colloidal techniques. Colloidalmixing involves the dispersion of the starting particles in a compatibleliquid medium. The dispersion may be electrostatic, steric, or acombination of the two depending on the surface characteristics of theparticles being dispersed. Mixing is then accomplished byultrasonication of the particle-fluid slip. Other mixing techniques maybe used but they are generally not as effective as ultrasonic methods.

The final step in controlling the packing morphology of the green orprefired body involves the actual consolidation of the particles into adesired shape. To do this, a method for removing the dispersion fluidmust be used. This can be as simple as filtering out the solid particlesprior to cold pressing them to shape, or it may involve the use of slipcasting or pressure casting techniques where the fluid is sucked orpushed out of the slip, respectively. Other, more advanced methods, suchas injection molding or extrusion of the starting constituents may alsobe used.

DENSIFICATION

Once the conditions for achieving the optimum packing morphology havebeen obtained, the second step, in the case of systems processed abovethe melting point of Gd (e.g., B₄ C-Gd cermets and Gd₂ O₃ -Gd cermets),is to apply the capillarity thermodynamic criteria of achieving rapiddensification through the kinetics of microstructural rearrangement vialiquid-phase sintering. Due to a greater degree of reactivity, it ismore difficult to obtain this rapid consolidation in the case of B₄ C-Gdcermets than it is in the case of Gd₂ O₃ -Gd cermets, and the formerrequires the application of pressure with temperature to externallyaccelerate the densification kinetics.

In Gd₂ O₃ -Gd cermets, the criterion of a low contact angle of theliquid Gd on the solid Gd₂ O₃ must be achieved. This condition is oftenreferred to as wetting. In B₄ C-Gd cermets, the external application ofpressure negates this requirement; however, any wetting of solid B₄ C byliquid Gd will assist the rearrangement process during pressing.

Wetting is defined as any process in which a solid-liquid interface isformed such that the measured angle through the molten liquid phase isacute. This is illustrated in FIG. 2A. Non-wetting is illustrated inFIG. 2B. The driving force for wetting is a reduction in free energy ofthe system, where the system is defined as the local solid, liquid, andvapor phases that coexist.

During wetting in B₄ C-Gd and Gd₂ O₃ -Gd cermets, chemicalnonequilibrium thermodynamic conditions exist between the solid, liquid,and vapor phases of the system. This nonequilibrium manifests itself inthe form of interfacial reactions that continue until a state ofchemical equilibrium is reached in the system.

Interfacial chemical reactions result in mass transfer across the B₄C-Gd or Gd₂ O₃ -Gd interfaces. Mass transfer results in a net decreasein the system free energy and usually begins and continues duringsintering until chemical equilibrium is achieved. This processultimately results in the formation of interfacial reaction products.

Contact-angle measurements can be made to quantify this wettingphenomenon. This is easily done by heating the Gd metal atop a polishedsubstrate of B₄ C or Gd₂ O₃. The contact angle is then measured in-situand recorded.

As an example, contact angle data for Gd on a B₄ C substrate ispresented in FIG. 3 which shows the contact angle as a function oftemperature in a vacuum environment of ≈10⁻⁴ torr. It is also importantto consider the time variable in making these measurements. Without thethree coupled variables; temperature, time, and pressure, it is notpossible to accurately replicate the conditions necessary to achieve adesired wetting condition. Processing temperatures for producing B₄ C-Gdcermets, under wetting conditions alone, are in the range of just above1200° C. to 1300° C. Processing times, due to the very reactive natureof this system, are typically less than 10 minutes.

It is interesting to note that wetting in the B₄ C-Gd system occursbelow the 1313° C. melting point of Gd. This must be due to existence ofa B-C-Gd ternary eutectic. There is no published phase equilibria forthis system; however, a 1180° C. binary eutectic is reported for theB-Gd system indicating a high-probability for this conclusion.

The time parameter is particularly important and the one that constrainsthe liquid-phase sintering of B₄ C-Gd and Gd₂ O₃ -Gd cermets the most.This is because mass transfer across the interfaces in these cermets istime dependent. The higher degree of reactivity in a B₄ C-Gd cermetindicates that mass transfer across its interface is greater/faster thanmass transfer across Gd₂ O₃ -Gd interfaces. The application of externalpressure to the sintering process using hot pressing or hot isostaticpressing changes the time scale for rearrangement and can allowdensification to occur much quicker.

The parameter of atmosphere is also important because it affects thevapor phase in the solid-liquid-vapor system. This becomes a significantissue when processing molten Gd at pressures below its vapor pressure.Published vapor pressure data for Gd is plotted in FIG. 4 and indicatesthat vacuum processing of material systems containing Gd could occurwithout evaporation of molten Gd at temperatures between 1200°-1300° C.(1473-1573K) when the atmospheric pressure remains at or above ≈10⁻²torr. Actual vacuum processing of systems containing molten Gd haveshown that the kinetics of evaporation for Gd are slow and thatprocessing at 10⁻⁴ torr is possible with minimal contamination to thefurnace.

The key point of the second step of this invention, as applied to B₄C-Gd and Gd₂ O₃ -Gd cermets, is that highly reactive systems like B₄C-Gd require the application of external pressure to achievedensification. This implies that squeeze casting, hot pressing or hotisostatic pressing will be required to densify these compositions. Inless reactive systems like Gd₂ O₃ -Gd, the application of pressure isnot required and complete densification can be achieved by pressurelesssintering or liquid-metal infiltration techniques.

In the case of systems processed below the melting point of Gd (e.g., B₄C-Gd or Gd₂ O₃ -Gd metal-matrix composites), the second step of theinvention is densification by establishing the correct plastic flowconditions such that the Gd metal will flow around the B₄ C or Gd₂ O₃ceramic phases.

The whole process is dependent on the work hardening, recovery,recrystallization, and grain growth of Gd metal in combination withparticles, whiskers, or fibers of B₄ C or Gd₂ O₃. A flow chart of thefabrication sequence for metal-matrix composites is given in FIG. 5.

B₄ C or Gd₂ O₃ material is mixed with atomized Gd powder in the desiredproportions either dry, or wet (colloidal) and then dried. The mixtureis then compacted at room temperature using axial or isostatic coldpressing methods. This compact is either vacuum hot pressed or inductionmelted into a billet and the billet subsequently extruded.

Vacuum hot pressing or induction melting is an operation where themetal+ceramic billet is taken to near the B-C-Gd ternary eutectictemperature (≈1200° C.) in the case of B₄ C-Gd metal-matrix composites,or to near the liquidus of Gd (≈1300° C.) in the case of Gd₂ O₃ -Gdmetal-matrix composites.

In the case of vacuum hot pressing, once the billet is pressed to fulldensity, the die is then cooled to a temperature below the solidus of Gd(approximately 100° C. below the liquidus or eutectic temperature) whilemaintaining pressure and vacuum, the billet removed from the die, andthen from the hot press. In the case of induction melting, themetal+ceramic mixture is heated in a vacuum induction furnace to atleast the melting point of Gd, inductively mixed in the molten state,allowed to cool, and then removed from the furnace and crucible.

After vacuum hot pressing or induction melting the billet is extrudedthrough a die at a temperature ≈75% of the eutectic or liquidustemperature. Other conventional metal working procedures may also becarried out on the billets. The extrusion of tubes, rods, plates, bars,and shaped sections are possible. Forging, rolling, and squeeze castingoperations are also feasible with B₄ C-Gd and Gd₂ O₃ -Gd metal-matrixcomposites.

Extrusion, forging, rolling, and casting pressures vary widely dependingon the process, but more importantly depending on the ceramic solidsloading of the composite being worked with, higher pressures beingrequired the greater the ceramic content. Nominal pressures are usedwith ceramic loadings up to 25 vol. %. Substantially higher pressuresare required up through 40 vol. % ceramic constituent and subsequent hotisostatic pressing may be required to densify metal matrix compositeswith initial ceramic contents between 40-50 vol. %.

The key point of the second step of this invention, as applied to B₄C-Gd and Gd₂ O₃ -Gd metal-matrix composites, is that highly reactivesystems like B₄ C-Gd require consolidation at temperatures near theB-C-Gd ternary eutectic temperature in order to maximize the plasticflow of the Gd matrix material around the B₄ C particles, fibers, orwhiskers. In less reactive systems like Gd₂ O₃ -Gd, the consolidationtemperature must be closer to the liquidus of the Gd matrix foroptimizing the plastic flow around Gd₂ O₃ particles, fibers, orwhiskers.

In the case of systems processed from refractory starting constituents(e.g., B₄ C-Gd₂ O₃ composites), the second step of this inventioninvolves producing the correct interphase diffusion and rearrangementconditions such that the composite can achieve a fully dense state.

This is accomplished primarily by solid-state diffusion; however,viscous flow and liquid diffusion are also potential contributors to theprocess. Solid-state sintering occurs readily between Gd₂ O₃ grains dueprimarily to the ionic nature of this phase. B₄ C, on the other hand,does not easily sinter due to its covalent nature and low volume andgrain boundary diffusion rates. Hence, B₄ C-Gd₂ O₃ systems that are Gd₂O₃ rich should be easier to sinter than systems that are B₄ C rich. Thislatter category, as well as the former, can be densified usingliquid-phase sintering techniques.

Sintering in the presence of a liquid phase is a way of bonding two ormore materials, which have different melting points, into dense bodieswithout complete fusion of the more refractory Gd₂ O₃ or B₄ C phases.This type of sintering is unique in that the system remains multiphasethroughout the entire process and the maximum temperature attained isbetween the liquidus and solidus of the system. The previously discussedliquid-phase sintering of cermets showed how Gd was used as the liquidphase. In the case of B₄ C-Gd₂ O₃ compositions, the Gd₂ O₃ phase oranother phase becomes the liquid.

There are basically two types of sintering that can occur in thepresence of a liquid phase. The first type, liquid-phase sintering, usesa material transport mechanism involving viscous flow and diffusion. Theother type, reactive liquid sintering, uses a material transportmechanism of viscous flow and solution precipitation. Both of thesemechanisms have the same driving force which evolves from capillarypressures and surface tensions occurring within the composite duringsintering.

It is widely accepted that the sintering of nonoxide ceramics, like B₄C, to theoretical density is only possible with the aid of certainadditions of impurities which result in the formation of a liquid phase.Without such additives it is necessary to apply pressure to the systemin order to force it to densify.

In Gd₂ O₃ -rich B₄ C-Gd₂ O₃ composites the primary driving force fordensification is the difference in free energy or chemical potentialbetween the free surfaces of Gd₂ O₃ particles and the points of contactbetween adjacent Gd₂ O₃ particles. This solid-state diffusion processbetween the Gd₂ O₃ particles involves material transport by surface andvolume diffusion. Surface diffusion does not result in densification;however, volume diffusion does. Volume diffusion occurs along grainboundaries and through lattice dislocations. Consequently, a system thatis rich in Gd₂ O₃ should sinter to near full density without theapplication of external pressure to the system. In addition, thereaction products that form between Gd₂ O₃ and B₄ C (e.g., GdBO₃, GdB₃O₆, B₂ O₃, etc.) may also serve as sintering aids to densification.

In B₄ C-rich B₄ C-Gd₂ O₃ composites, just the opposite occurs. There arenot enough free surfaces and points of contact with respect to the Gd₂O₃ phase and consequently densification is severely inhibited. This isquickly overcome, however, by use of hot pressing or hot isostaticpressing methods.

Gd₂ O₃ melts at ≈2320° C., forms a gadolinium borate (GdBO₃) that meltsat ≈1590 .C, and forms a binary eutectic with B₂ O₃ at ≈1230° C. B₄ C,on the other hand, melts at ≈2350° C. but can form a B203 surface layerin the presence of oxygen at much lower temperatures. This sharedcharacteristic between Gd₂ O₃ and B₄ C assists in the densification ofB₄ C-Gd₂ O₃ composites by providing a low temperature sintering aid forthe system. FIG. 6 shows the phase equilibria for the GdBO₃ -B₂ O₃pseudobinary system. The formation of a low melting point borate andeutectic clearly indicates that liquid-phase densification is possiblein B₄ C-Gd₂ O₃ composites.

The key point of the second step of this invention, as applied to B₄C-Gd₂ O₃ composites, is that Gd₂ O₃ -rich systems do not require theapplication of external pressure for densification while B₄ C-richsystems do.

Reaction Thermodynamics

If molten-metal liquid-phase sintering is to occur, the B₄ C-Gd or Gd₂O₃ -Gd mixture must satisfy the reaction-thermodynamic criterion thatthe solid B₄ C phase and any metastable gadolinium-borocarbide,gadolinium-boride, gadolinium-carbide compounds or solid solutions bepartially soluble in the liquid Gd or alloy phases present; or that thesolid Gd₂ O₃ phase and any metastable gadolinium-oxide compounds orsolid solutions be partially soluble in the liquid Gd or alloy phasespresent.

If solid-state sintering and/or liquid-phase sintering is to occur in B₄C-Gd₂ O₃ composites, then the B₄ C-Gd₂ O₃ mixture must satisfy thereaction-thermodynamic criterion that the solid B₄ C and Gd₂ O₃ phasesand any metastable gadolinium-borocarbonate, gadolinium-borate,gadolinium-carbonate, gadolinium-borocarbide, gadolinium-boride,adolinium-carbide, gadolinium-oxide compounds or solid solutions bepartially soluble in the liquid GdBO₃, liquid B₂ O₃, or other liquidphases present.

To fully understand how a particular B₄ C-Gd, Gd₂ O₃ -Gd, or B₄ C-Gd₂ O₃composite will react at different processing isotherms, one mustconsider both thermodynamic and kinetic issues for each respectivecomposition. Merely examining phase-equilibria data is often times notenough. This is because reactive compositions typically form metastableinterfacial phases. In many cases, the phase diagrams are not availableanyway.

Hence, detailed studies using x-ray diffraction and opticalmetallographic equipment must be employed. The studies on eachrespective system need to be done in incremental steps, evaluating thecomposition as a function of processing history. Only in this way, canmetastable phases and equilibrium reaction products be correctlyidentified in a detailed fashion.

In a more general sense, however, its is possible to determine the rangeof compositions that are obtained from each respective system. Thecomposition ranges obtained for the B₄ C-Gd, Gd₂ O₃ -Gd, and B₄ C-Gd₂ O₃composites of this invention are presented in Tables I-III.

TABLE I. Semi-quantitative x-ray diffraction analysis of B₄ C-Gdcomposites.

For Cermets (measured)

Major phase: New unknown phase(s)

Secondary phases: GdB₄, GdC₂, B₈ C(?)

Minor phases: GdBC, Gd₂ C₃, Gd, B₄ C, B₁₃ C₂, GdB₆₆, free boron(?)

Trace phases: Gd₂ B₅, B₂₅ C, GdB₂ (?), GdB₆ (?), C(?)

For Metal-Matrix Composites (estimated)

Major phase: Gd

Secondary phases: New unknown phase(s)

Minor phases: GdB₄, GdC₂, B₈ C(?)

Trace phases: GdBC, Gd₂ C₃, Gd, B₄ C, B₁₃ C₂, GdB₆₆, free boron(?)

TABLE II. Semi-quantitative x-ray diffraction analysis of Gd₂ O₃ -Gdcomposities.

For Cermets (measured)

Major phase: Gd₂ O₃ (gadolinium III oxide)

Secondary phase: Gd

Minor phases: Gd₂ O₃ (gadolinium oxide), GdO(?)

Trace phases: Gd₂ O₃ (high-temperature, metastable)

For Metal-Matrix Composites (estimated)

Major phase: Gd

Secondary/Minor phase: Gd₂ O₃ (gadolinium III oxide)

Trace phases: Gd₂ O₃ (gadolinium oxide), GdO(?)

TABLE III. Semi-quantitative x-ray diffraction analysis of B₄ C-Gd₂ O₃composites.

For Gd₂ O₃ -rich Composites (measured/estimated)

Major phase: Gd₂ O₃

Secondary phase: B₄ C

Minor/Trace phases: GdBO₃, GdB₃ O₆ (?), B₂ O₃ (?), New unknown phase(s),other B₄ C-Gd phases (?)

For B₄ C-rich Composites (measured/estimated)

Major phase: B₄ C

Secondary phase: Gd₂ O₃

Minor/Trace: New unknown phase(s), GdB₄, GdC₂, B₈ C(?), GdBO₃, GdB₃ O₆(?), B₂ O₃ (?), other B₄ C-Gd phases

Tables I and III indicate the confirmation of a new composition ofmatter. It is noted as "New unknown phase(s)" in the tables because morethan one new phase may be present. At this time, the x-ray diffractionpeaks associated with this finding also may actually indicate (1) only aportion of the diffraction peaks, i.e., some peaks may be masked by thepresence of larger overlapping peaks; and (2) the existence of otherphases' peaks may accidentally be included in the reported peaks below.Table IV gives the approximate peak locations (d-spacing) and estimatedpeak height (I/I_(o)).

                  TABLE IV                                                        ______________________________________                                        Approximate d-spacing and estimated intensity of                              x-ray diffraction peaks for the new phase(s)                                  present in B.sub.4 C--Gd and B.sub.4 C--Gd.sub.2 O.sub.3 composites.          d-spacing (Angstroms)                                                                          Intensity (I/I.sub.o)                                        ______________________________________                                        8.0870            100                                                         5.7306           ≦85                                                   3.5660           ≦41                                                   3.7837           ≦38                                                   1.7299           ≦35                                                   1.7094           ≦34                                                   1.2942            33                                                          2.1944           ≦33                                                   1.9714            33                                                          1.0262            33                                                          1.2423            33                                                          1.3176            32                                                          1.7747            31                                                          ______________________________________                                    

It is anticipated that the new phase(s) is/are ternary gadoliniumborocarbide(s).

Phases shown in Tables I-III with a (?) indicate that the existence ofthis phase is likely; however, there is not enough evidence, based onthe current analysis, to say for sure if it is really present or not.

It is important to point out that subsequent heat treatment of all thecompositions of this invention will result in diminishing the majorindicated phases and promoting the growth of some of the less prevalentphases (secondary, minor, and/or trace phases).

Examples of some refractory neutron absorbing compositions of matter, inaccordance with the invention, are given in FIGS. 7-9. FIGS. 7, 8, and 9show the microstructures of a B₄ C-Gd cermet, a B₄ C-Gd₂ O₃ composite,and a Gd₂ O₃ -Gd cermet, respectively.

EXAMPLE 1

Start with -325 mesh Gd powder and -325 mesh B₄ C powder. Weigh out a 60vol. % B₄ C and 40 vol. % Gd sample. Ultrasonically mix the two powderstogether in a methanol slip. Filter out the methanol and dry back themixture. Pour the mixture into a boron nitride lined, reinforcedgraphite punch and die assembly. Place assembly with mixture in a vacuumhot press and apply 3 ksi pressure to punches. Rapidly heat to 1250° C.in a vacuum of ≈10⁻⁴ torr. On passing through 1200° C. increase appliedpressure to 10 ksi. Hold at 1250° C. until pressing action ceases.Furnace cool part under vacuum while maintaining 10 ksi. At 1000° C.reduce applied pressure to 3 ksi and continue cooling to roomtemperature. Remove part from assembly. Result: B₄ C-Gd cermetcomprising

    ______________________________________                                        New unknown phase(s)   47 vol. %                                              GdB.sub.4              21 vol. %                                              GdC.sub.2              19 vol. %                                              other, per Table I     13 vol. %                                              ______________________________________                                    

EXAMPLE 2

Start with -325 mesh Gd powder and 10 μm Gd₂ O₃ powder. Weigh out a 75vol. % Gd₂ O₃ and 25 vol. % Gd sample. Press only the Gd₂ O₃ powder in asteel punch and die assembly at 10 ksi. Remove the porous Gd₂ O₃ compactfrom the die and place it inside the bed of the Gd powder resting in agraphite lined tungsten crucible. Cover the crucible and place completeassembly in a vacuum furnace. Rapidly heat to 1350° C. under a vacuum of≈10⁻⁴ torr. Hold at 1350° C. for 10 minutes to allow molten Gd metal toinfiltrate into the Gd₂ O₃ preform. Furnace cool to room temperature andremove solidified melt from the crucible. Machine away any residual Gdmetal that did not infiltrate. Result: Gd₂ O₃ -Gd cermet comprising

    ______________________________________                                        Gd.sub.2 O.sub.3 (III)                                                                              68 vol. %                                               Gd                    22 vol. %                                               other, per Table II   10 vol. %                                               ______________________________________                                    

EXAMPLE 3

Start with -40 mesh Gd powder and -325 mesh B₄ C powder. Weigh out a 20vol. % B₄ C and 80 vol. % Gd sample. Mechanically vibrate the mixturefor 5 minutes. Pour the mixture into a steel/graphite punch and dieassembly and compact at 20 ksi. Place the assembly with mixture into avacuum hot press and heat to 1200° C. under a vacuum of ≈10⁻³ torr.Maintain 7 ksi during heat up and hold at 1200° C. until compactionceases. Cool at 50° C. per minute under vacuum while maintaining 7 ksiapplied pressure. On passing down through 900° C. reduce appliedpressure to 3 ksi. Cool to room temperature and remove billet. Extrudebillet with an extrusion ratio of 1:10 at a temperature of 980° C.Finally, heat treat extruded part for 3 hours at 1100° C. Result: B₄C-Gd metal-matrix composite comprising

    ______________________________________                                        Gd                     55 vol. %                                              New unknown phase(s)   28 vol. %                                              GdB.sub.4               8 vol. %                                              GdC.sub.2               6 vol. %                                              other, per Table I      3 vol. %                                              ______________________________________                                    

EXAMPLE 4

Start with -40 mesh Gd powder and 10 μm Gd₂ O₃ powder. Weigh out a 15vol. % Gd₂ O₃ and 85 vol. % Gd sample. V-blend the powders together for30 minutes. Place the mixture in a susceptor crucible and place in aninduction melting furnace. Inductively heat the assembly at 1400° C. ina vacuum of 10⁻² torr for 1 hour. Furnace cool to room temperature andremove billet from crucible. Extrude billet at an extrusion ratio of 1:8at a temperature of 900° C. Finally, heat treat the part at 1000° C. for2 hours. Result: Gd₂ O₃ -Gd metal-matrix composite comprising

    ______________________________________                                        Gd                    64 vol. %                                               Gd.sub.2 O.sub.3 (III)                                                                              25 vol. %                                               other, per Table II   11 vol. %                                               ______________________________________                                    

EXAMPLE 5

Start with -20 μm B₄ C powder and -10 μm Gd₂ O₃ powder. Weigh out a 65vol. % B₄ C and 35 vol. % Gd₂ O₃ sample. Prepare a sterically stabilizedcolloidal suspension and ultrasonicate the slip for 3 minutes. Pressurecast the slip at 80 psi for 24 hours. Remove green part from thepressure caster and place in a graphite punch and die assembly. Placethe assembly in a hot press. Hot press the green body at 1950° C. withan applied pressure of 5 ksi in flowing argon. Cool at 25° C. per minutedown through 500° C. while maintaining 5 ksi applied pressure. Onpassing 500° C. cool at 100° C. per minute while maintaining only 3 ksiapplied pressure. Remove part from die at room temperature. Result: B₄C-rich B₄ C-Gd₂ O₃ composite comprising

    ______________________________________                                        B.sub.4 C             62 vol. %                                               Gd.sub.2 O.sub.3      33 vol. %                                               other, per Table III   5 vol. %                                               ______________________________________                                    

EXAMPLE 6

Start with 1-3 μm B₄ C powder and -10 μm Gd₂ O₃ powder. Weigh out a 30vol. % B₄ C and 70 vol. % Gd₂ O₃ sample. Apply electrostatic dispersiontechniques to obtain a slip. Ultrasonicate slip for 5 minutes in an icebath. Slip cast slurry into plaster mold. Remove green part and vacuumdry at 100° C. and 50 millitorr for 12 hours. Place green part in inertgas furnace and fire at 1850° C. for 1 hour. Cool at 50° C. per minuteto room temperature. Result: Gd₂ O₃ -rich B₄ C-Gd₂ O₃ compositecomprising

    ______________________________________                                        Gd.sub.2 O.sub.3      66 vol. %                                               B.sub.4 C             27 vol. %                                               other, per Table III   7 vol. %                                               ______________________________________                                    

OTHER APPLICABLE COMPOSITES

Although the best compositions for neutron absorption are those using Gdmetal (or alloy), upon which this invention is based, there are severalother metals that may be combined with B₄ C or Gd₂ O₃ to form othercermet and metal-matrix compositions, which may be useful for the sameor other applications. The metals that can be used in accordance withthe methods of this invention are Cd, In, Te, Pb, Ce, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and alloys thereof.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the scope of the invention whichis intended to be limited only by the scope of the appended claims.

We claim:
 1. A method of forming a neutron absorbing refractorycomposite material, comprising:selecting a pair of initial reactantsfrom the group consisting of: (a) gadolinium or alloys thereof, (b)boron carbide, and (c) gadolinium oxide; consolidating the initialreactants; densifying the consolidated initial reactants; reacting thedensified initial reactants to produce a material of preselectedcomposition having a plurality of interfacial reaction product phases.2. The method of claim 1 wherein the initial reactants are in the formof particles, fibers, or whiskers.
 3. The method of claim 1 wherein thestep of consolidating the initial reactants comprises:selecting startingparticle size distributions; dispersing and mixing the particles;consolidating the dispersed and mixed particles substantiallyhomogeneously.
 4. The method of claim 1 for producing a B₄ C-Gd or Gd₂O₃ -Gd cermet, wherein the step of densifying the initial reactantscomprises wetting the B₄ C or Gd₂ O₃ phase by the Gd metal phase.
 5. Themethod of claim 1 for producing a B₄ C-Gd or Gd₂ O₃ -Gd metal-matrixcomposite, wherein the step of densifying the initial reactantscomprises producing plastic flow of the Gd metal phase around the B₄ Cor Gd₂ O₃.
 6. The method of claim 1 for producing a B₄ C-Gd₂ O₃composite wherein the step of densifying comprises producing interphasediffusion and rearrangement.
 7. The method of claim 1 wherein the stepof reacting is performed by heating at a preselected temperature for apreselected time.
 8. The method of claim 3 wherein the step of selectingstarting particle size distributions comprises selecting a plurality ofdifferent particle sizes to enhance particle packing density.
 9. Themethod of claim 3 wherein the step of dispersing and mixing is performedby colloidal dispersion and ultrasonic mixing.
 10. The method of claim 3wherein the step of consolidating is selected from:(a) filteringfollowed by cold pressing, (b) slip casting, (c) pressure casting. 11.The method of claim 1 wherein the step of consolidating is performed byinjection molding or extruding the initial reactants.
 12. The method ofclaim 1, for producing a B₄ C-Gd cermet, further comprising applyingexternal pressure during the densification step.
 13. The method of claim12 comprising performing the step of densification by squeeze casting,hot pressing, or hot isostatic pressing.
 14. The method of claim 1, forproducing a B₄ C-Gd cermet, wherein the densification step is performedat a temperature of about 1200° C.-1300° C. for a time of less thanabout 10 minutes.
 15. The method of claim 4, for producing a Gd₂ O₃ -Gdcermet, wherein the densification step is performed by pressurelessliquid-phase sintering or liquid-metal infiltration.
 16. The method ofclaim 5, for producing a B₄ C-Gd metal-matrix composite, wherein thedensification step is performed at about the B-C-Gd ternary eutectictemperature (1200° C.).
 17. The method of claim 5, for producing a Gd₂O₃ -Gd metal-matrix composite, wherein the densification step isperformed at about the liquidus temperature of Gd (1300° C.).
 18. Themethod of claim 5 wherein the densification step is performed by vacuumhot pressing or induction melting.
 19. The method of claim 6, forproducing a Gd₂ O₃ -rich composite, wherein the densification step isperformed by solid-state diffusion.
 20. The method of claim 6, forproducing a B₄ C-rich composite, wherein the densification step isperformed by liquid-phase sintering.
 21. The method of claim 20 furthercomprising applying external pressure during the densification step. 22.The method of claim 21 wherein the densification step is performed byhot pressing or hot isostatic pressing.
 23. A method of forming aceramic-metal composite material comprising:selecting a ceramic phasefrom B₄ C and Gd₂ O₃, and a metal phase from Cd, In, Te, Pb, Ce, Pr, Nd,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and alloys thereof;consolidating the ceramic phase and metal phase; densifying theconsolidated ceramic and metal phases; reacting the densified ceramicand metal phases to produce a plurality of interfacial reaction productphases.